1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image formed on a recording medium onto the recording medium, as well as to an image forming apparatus having the fixing device.
2. Description of the Related Art
An electrophotographic image forming apparatus for forming images by means of electrophotography, such as a copying machine and a printer, is provided with a fixing device. As the fixing device, as shown in FIG. 2 for explaining the invention which will hereafter be described, there is known a fixing device 7 having a heat roller 71 which is heated by a heating portion and a pressure roller 72 which is brought into contact with the heat roller 71 under a predetermined contact pressure. In the fixing device 7, a recording medium, for example, a recording sheet 202 having a yet-to-be heated developer e.g. toner T transferred on its image surface i.e. heat-roller 71-side surface 202a by a transfer device located anteriorly of the fixing device 7, is caused to pass through a region between the heat roller 71 and the pressure roller 72, whereupon the yet-to-be heated developer is fixed onto the recording sheet 202.
In order to convey the recording medium, the fixing device 7 has a post-fixing roller 75 and a post-fixing driven roller element 76 which rotates depending on the rotation of the post-fixing roller 75 that are located posteriorly of the heat roller 71 in a direction in which the recording medium is conveyed; that is, located downstream from the heat roller 71 in the conveyance direction. As the post-fixing roller 75 is rotated, with the recording medium 202 caught in the space between the post-fixing roller 75 and the post-fixing driven roller element 76, these rollers convey the recording medium 202.
As shown in FIG. 4 for explaining the invention which will hereafter be described, in the heat roller 71, a key 222 formed in a fixing roller driving gear 220, which is a gear wheel of the fixing roller, is fitted to a keyway 221 formed in a journal 200 extending from a roller main body 201, whereby the fixing roller driving gear 220 and the roller main body 201 are coupled to each other and are thus allowed to rotate together.
In order to rotate the heat roller 71 and the post-fixing roller 75, as shown in FIG. 3 for explaining the invention which will hereafter be described, a paper discharge unit-side gear 211, which is a driving gear wheel engageable with the fixing roller driving gear 220, is rotated by a non-illustrated motor, thus causing the fixing roller driving gear 220 to rotate. The driving force exerted by the motor is transmitted from the paper discharge unit-side gear 211, through the fixing roller driving gear 220, to the heat roller 71. The post-fixing roller 75 is rotated under the driving force transmitted from the fixing roller driving gear 220, through a first coupling gear 230 and a second coupling gear 231, to a post-fixing roller driving gear 232.
Returning to FIG. 2, the heat roller 71 and the pressure roller 72 are each constructed of an elastic roller. Therefore, in a case where the recording medium 202 is of the type that has a certain degree of thickness, such as gloss paper or heavy paper, upon the passage of the recording medium 202 through a nip region 203, the heat roller 71 and the pressure roller 72 undergo elastic deformation owing to the recording medium 202, in consequence whereof there results a buildup of elastic energy in the rollers 71 and 72. The elastic energy is released when the recording medium 202 comes out of the nip region 203, thus causing speeding up of the heat roller 71 and the pressure roller 72. With the resultant accelerated rotation of the heat roller 71, the fixing roller driving gear 220 shown in FIG. 3 is rotated after a moment of speeding up, with the result that so-called tooth jumping occurs between the fixing roller driving gear 220 and the paper discharge unit-side gear 211.
Upon the re-engagement between the fixing roller driving gear 220 and the paper discharge unit-side gear 211, the recording medium 202 is returned to a normally conveyed state where the tooth jumping has not occurred as yet. However, as shown in FIG. 6 which will hereafter be explained, until such time that the fixing roller driving gear 220 and the paper discharge unit-side gear 211 engage with each other, the recording medium 202 is standing still, with its upstream-side end in the conveyance direction A (hereafter referred to as “tail end”) 202b kept at a position to which it is moved immediately after coming out of the nip region 203. During that time, inconveniently, the post-fixing driven roller element 76 kept in contact with the image surface 202a leaves some impressions on the image.
A technique to prevent jumping of gear teeth, although it is not related to fixing device development, is disclosed in Japanese Unexamined Patent Publication JP-A 9-286529 (1997). JP-A 9-286529 discloses a sheet feeding apparatus. In the sheet feeding apparatus disclosed in JP-A 9-286529, when a paper-feeding roller is put under a load which is greater than a predetermined level, the engagement between the paper-feeding roller and a driving shaft is released so that only the driving shaft is rotated, that is, the driving shaft is idled. This makes it possible to reduce the load applied to the driving gear and thereby prevent occurrence of tooth jumping in the driving gear.
When, in the fixing device, the driving shaft is idled to prevent occurrence of tooth jumping in the fixing roller driving gear corresponding to the driving gear as in the technique disclosed in JP-A 9-286529, the conveyance of the recording medium will be brought to a stop. Therefore, just as in the case where the tooth jumping occurs, the post-fixing driven roller element leaves some impressions on the image surface. The technique disclosed in JP-A 9-286529 is thus not applicable to the fixing device.